At their father's tomb, you get to see Sesshoumaru's true form for the first time. First, a clear, wind type of tornado surrounds him, which makes his kimono, hair and pelt wave in the air high up. His magenta stripes get jagged, his golden eyes go small and so does his pupils. Then his eyes turn red with blue-green pupils, then the pupils become slits and his eyes look more dog/demon like and his face now looks like a dog with his magenta strips joining around the jaw. He then glows pink and shoots up in the air and bounces around. When he lands, he is a 55ft or 60ft white and silver dog with floppy white ears with fire looking fur behind his paws. His tail is big and his whole fur is fluffy looking, it makes you want to glomp him forever in that form.
Also in this transformation, his mouth can produce the poisonous gas that was in his right hand. What he does is, bite down on something or the ground and he lets it drop on the floor and the poisonous gas goes in the room, making it have dark greenish poison clouds. Humans and demons, no matter what they are or how powerful, they can't stay in those fumes for long but it doesn't hurt Sesshoumaru.